1. Field of the Invention
The present invention relates generally to lithium batteries. More particularly, it concerns a method of preparing a spinel oxide suitable for use as a cathode in a lithium battery. The spinel oxide, Li4Mn5O12, is synthesized via a solution phase oxidation reaction followed by oven firing.
2. Description of Related Art
The high cost and high toxicity of cobalt has created enormous interest in development of less expensive, environmentally benign manganese-based cathodes as an alternative to cobalt-based cathodes for rechargeable lithium batteries. The spinel oxide LiMn2O4 is being intensively pursued in this regard (Thackeray et al., 1983; Ohzuku et al., 1990; Thackeray et al., 1992; Tarascon et al., 1991; Gummow et al., 1994; Ferg et al., 1994; Yamada et al., 1995; Gao and Dahn, 1996a).
LiMn2O4 shows two plateaus in voltage versus capacity plots, one around 4V and the other around 3V. While the 4V region generally shows good cyclability and ample capacity, the 3V region exhibits drastic capacity fading upon cycling due to the macroscopic volume change associated with a cooperative Jahn-Teller distortion. As a result, the capacity in the 3V region (about 150 mAh/g, theoretically) of the stoichiometric LiMn2O4 spinel cannot be practically utilized.
It is known that the cyclability in the 3V region can be improved by increasing the average oxidation state of manganese through a substitution of lithium for manganese in Li1+xMn2O4. Such substitutions may help to suppress Jahn-Teller distortions. For example, in the 3V region, the limiting case of x=0.33, corresponding to composition Li4Mn5O12 is known to show better cyclability than the x=0 case corresponding to composition LiMn2O4. Although the increase in the oxidation state of manganese leads to a monotonic decrease in capacity in the 4 V region, it results in a better cyclability in the 3 V region, as cubic symmetry can be preserved to higher degrees of lithium insertion into the manganate lattice sites.
Li4Mn5O12 may be unstable to heat treatment and may disproportionate to LiMn2O4 and Li2MnO3 at higher temperatures (Thackeray et al., 1992; Thackeray, Mansuetto, Dees and Vissers, 1996; Gao and Dahn, 1996b). This is significant in that Li4Mn5O12 is generally synthesized by firing a mixture of manganese oxides or salts with lithium salts, and the diffusional limitations in such reactions necessitate higher firing temperatures (T greater than 600xc2x0 C.) in order for the reaction to go to completion.
Also, it is known that the oxidation state of manganese in the raw materials used to prepare spinel oxide materials is an important factor in determining the nature of the reaction product. For example, while raw materials with Mn3+ tend to favor the formation of LiMn2O4, those with Mn4+ help to stabilize Li4Mn5O12.
The present invention provides low temperature synthesis procedures for the preparation of Li4Mn5O12. The solution phase oxidation reaction disclosed herein employs lithium peroxide with lithium hydroxide and manganese acetate to obtain a precursor containing Mn4+. This precursor is preferably in the form of a precipitate. Advantageously, the use of a precursor containing Mn4+ may favor the formation of Li4Mn5O12 at low temperatures. In the method of the present invention, the precursor containing Mn4+ may be fired at low to moderate temperatures (up to about 500xc2x0 C.), causing it to lose water and yield Li4Mn5O12. Preferably the precursor is fired at temperatures of about 500xc2x0 C. or less. Advantageously, the low firing temperatures of the present method may serve to preclude disproportionation of Li4Mn5O12 to form LiMn2O4 and Li2MnO3. As used herein, xe2x80x9clow temperaturexe2x80x9d means a temperature of about 500xc2x0 C. or less. As used herein, a precursor that is xe2x80x9cfiredxe2x80x9d is heated. As used herein, a substance, such as a precursor, having a first temperature may be xe2x80x9cheatedxe2x80x9d or may undergo xe2x80x9cheatingxe2x80x9d by causing the temperature of the substance to rise relative to the first temperature.
Alternatively, hydrogen peroxide can be used instead of lithium peroxide and lithium carbonate can be used instead of lithium hydroxide with modifications in quantity that would be apparent to one of skill in the art. Advantageously, the general procedure described can be used with appropriate modifications, such as the use of manganese acetate with or without other metal acetates, that would be apparent to one of skill in the art to obtain other spinel cathodes such as Li2Mn4O9xe2x88x92xcex4(0xe2x89xa6xcex4xe2x89xa61), Li1+xMn2xe2x88x92xO4+xcex4(0xe2x89xa6xxe2x89xa60.33and 0xe2x89xa6xcex4xe2x89xa60.5)and Li1+xMn2-x-yMyO4+xcex4(0xe2x89xa6xxe2x89xa60.33, 0xe2x89xa6yxe2x89xa62.0, 0xe2x89xa6xcex4xe2x89xa60.5 and M=Cr, Fe, Co, Ni or Cu). The general procedure may also be used to produce other transition metal oxide cathodes such as LiCoO2, LiNiO2 and LiNi1xe2x88x92yMyO2 (M=Mn, Fe, Co or Cu). In this case, the procedure calls for the use of cobalt acetate or nickel acetate with or without other metal acetates, and a firing temperature of about 300xc2x0 C. to about 900xc2x0 C.
Surprisingly, the solution-based, low-temperature method described herein is able to access all Mn4+ without oxygen vacancies in Li4Mn5O12. Samples synthesized according to the methods disclosed herein at Txe2x89xa6500xc2x0 C. show excellent capacity retention in the 3V region with a maximum capacity of 160 mAh/g, which is close to the theoretical value.
It is contemplated that the materials prepared via the methods of the present invention may be useful as cathodes for rechargeable lithium batteries. It is further contemplated that the materials prepared via the methods of the present invention may be particularly well-suited for use in lithium polymer batteries. Advantageously, the Li4Mn5O12 samples obtained by this low temperature approach exhibit a capacity close to the theoretical value, with excellent cyclability, making them well-suited for use as a cathode in a rechargeable lithium battery. The electrochemical characteristics of the samples also suggest possible use in electrochemical capacitor (supercapacitor) applications.
In a broad aspect, the invention is a process for forming a precipitate including admixing a first aqueous solution and a second aqueous solution with a third aqueous solution to produce the precipitate. As used herein, xe2x80x9cadmixingxe2x80x9d means mixing or blending using any suitable means such as stirring, vibrating, shaking, agitating or the like. The first aqueous solution may include lithium peroxide or hydrogen peroxide. The second aqueous solution may include lithium hydroxide or lithium carbonate. The third aqueous solution may include manganese acetate.
In other aspects, the process may include filtering the precipitate and heating the precipitate to produce a spinel oxide. The spinel oxides disclosed herein are transition metal oxides. The process may also include grinding the spinel oxide to form a cathode. The admixing may include stirring. The precipitate may be heated to about 500xc2x0 C. or less, and it may be heated at a rate of about 1xc2x0 C./minute to about 10xc2x0 C./minute. The precipitate may be heated to from about 300xc2x0 C. to about 500xc2x0 C., and it may be heated for about one to about five days. The precipitate may be allowed to dry in air at ambient temperature prior to being heated. The third aqueous solution may include manganese acetate without other metal acetates, and the spinel oxide may include Li4Mn5O12; Li2Mn4O9xe2x88x92xcex4, where 0xe2x89xa6xcex4xe2x89xa61; or Li1+xMn2xe2x88x92xO4+xcex4, where 0xe2x89xa6xxe2x89xa60.33 and 0xe2x89xa6xcex4xe2x89xa60.5. The third aqueous include manganese acetate with other metal acetates, such as chromium acetate, iron acetate, cobalt acetate, nickel acetate or copper acetate, for example, and the spinel oxide may include Li1+xMn2-x-yMyO4+xcex4, where 0xe2x89xa6xxe2x89xa60.33, 0xe2x89xa6yxe2x89xa62.0, 0xe2x89xa6xcex4xe2x89xa60.5 and M selected from the group consisting of Cr, Fe, Co, Ni and Cu. The first aqueous solution may include lithium peroxide, the second aqueous solution may include lithium hydroxide, and the precipitate may include LixMnyxe2x88x92xcex7Mxcex7Ozxc2x7nH2O, where 1xe2x89xa6xxe2x89xa61.33, 1.66xe2x89xa6yxe2x89xa62.0, 0xe2x89xa6xcex7xe2x89xa6y, 2.3xe2x89xa6zxe2x89xa64.5 and 0xe2x89xa6nxe2x89xa630.
In another aspect, the invention is a process for forming a precipitate including admixing a first aqueous solution and a second aqueous solution with a third aqueous solution to produce the precipitate. The first aqueous solution may include lithium peroxide or hydrogen peroxide. The second aqueous solution may include lithium hydroxide or lithium carbonate. The third aqueous solution may include cobalt acetate or nickel acetate.
In other aspects, the process may include filtering the precipitate and heating the precipitate to produce a transition metal oxide. The process may also include grinding the transition metal oxide to form a cathode. The precipitate may be heated to from about 300xc2x0 C. to about 900xc2x0 C. The precipitate may also be heated to from about 600xc2x0 C. to about 900xc2x0 C. The admixing may include stirring. The third aqueous solution may include cobalt acetate without other metal acetates, the transition metal oxide may include LiCoO2, and the precipitate may be heated for about one to about five days. The third aqueous solution may include nickel acetate without other metal acetates, the transition metal oxide may include LiNiO2, and the precipitate may be heated for about one to about five days. The third aqueous solution may include nickel acetate with other metal acetates, such as manganese acetate, iron acetate, cobalt acetate, or copper acetate, for example, the spinel oxide may include LiNi1xe2x88x92yMyO2, where 0xe2x89xa6yxe2x89xa61.0, and M is selected from the group consisting of Mn, Fe, Co, and Cu, and the precipitate may be heated for about one to about five days.